1. Field of the Invention
This invention relates to a disk storage system particularly used as a hard disk drive and employing a magnetoresistive (MR) head as a read head.
2. Description of the Related Art
A disk storage system, such as a hard disk drive (HDD), magnetically records data on a disk as a recording medium, and reproduces original record data from data read from the disk, using a magnetic head (hereinafter referred to simply as "head").
In the field of HDDs, a technique for using a magnetoresistive (MR) head as a read head for reading data from a disk has recently been developed to realize high record density. The HDD requires a read/write head for recording and reproducing data. In general, a head of a record/reproduction separated type, which consists of an MR head (as a read head) and an inductive head (as a write head) formed integral with the MR head as one body, is used as the read/write head. In this record/reproduction separated type head, the magnetic gap of the write head and that of the MR head can be optimized, and hence both record characteristics and reproduction resolution can be enhanced.
The MR head uses an element with a resistance, which is variable in accordance with a variation in magnetic flux corresponding to a variation in recording magnetic field on a disk. The data reproduction system of the HDD converts a variation in MR head resistance to a voltage signal corresponding to a read signal. In the HDD, a head slider on which the MR read head and the write head are mounted moves over the disk, with a fine flying height, for example, of about 50 nm kept above the disk. This being so, only a slight change in flying height may well cause collision of the MR head attached to the head slider, against a projection, etc. formed on the disk.
It is confirmed that the temperature of the MR head will abruptly increase when the head has collided against the surface of the disk, and the resistance of the head will accordingly change greatly. In accordance with an abrupt change in resistance of the MR head, the waveform of a read signal output from the MR head varies. This phenomenon is called "thermal asperity (TA)" phenomenon. FIG. 10A shows the characteristics of the TA phenomenon. In this figure, variations in resistance of the MR head appearing when the TA phenomenon occurs are indicated by variations in voltage level. FIG. 10B shows a normal read signal waveform corresponding to data undisturbed by the thermal asperity FIG. 10C shows a read signal waveform obtained when the thermal asperity phenomenon occurs.
As regards the thermal asperity (TA) phenomenon, see the document "MAGNETIC RECORDING CHANNEL FRONT-ENDS by K. B. Klaassen" (IEEE TRANSACTIONS ON MAGNETS Vol. 27, No. Nov. 6, 1991).
When the TA phenomenon has occurred, a read signal with an abnormal amplitude due to the TA, as shown in FIG. 10C, is input to the data reproduction section of the HUD. Since the amplitude of the read signal thus varies significantly, the data reproduction section cannot reproduce data until the amplitude of the read signal returns to a normal level. Moreover, an AGC circuit incorporated in the data reproduction section for keeping the level of the read signal constant will adversely be affected by the abnormal amplitude of the read signal. As a result, even if the output level of the MR head returns to its normal value, the output of the AGC circuit cannot be recovered for a certain period of time corresponding to the time constant of the abrupt change of the signal amplitude.
To solve the above problem, the aforementioned document, etc. proposes a method for detecting a change in DC level of the read signal output from the MR head, and adding a signal waveform of an opposite phase to the level-changing read signal to compensate the same. When the TA phenomenon occurs at the MR head, the amplitude of the read signal varies along an envelope as shown in FIG. 10C, because of abrupt increase in temperature and heat radiation. A DC level detection circuit for detecting the envelope of the signal waveform can be used as a TA detection circuit. Such a TA detection circuit is generally employed in a head amplifier circuit in the data reproduction system of the HDD.
As described above, in the HDD using the MR head, it is highly possible that the thermal asperity (TA) phenomenon will occur when the MR head collides against a projection (part of the disk surface itself or a fine particle attached thereto). The TA phenomenon will cause an abnormal change in waveform of the read signal output from the MR head, with the result that it is highly possible that a read error will occur during data reproduction. The aforementioned method is proposed to avoid this. The method, however, requires the DC level detection circuit (i.e. TA detection circuit) and a DC cancel circuit for performing addition of an opposite-phase signal. Furthermore, when in this method, the DC level of the read signal has abruptly changed and approached a signal component band, sufficient DC cancel cannot be performed.